How to Make It
Put the potatoes in a medium saucepan, cover with cold water and bring to a boil. Simmer over moderate heat until tender, about 25 minutes. Drain and let cool slightly. Peel the potatoes and cut them into 1/2-inch dice.
Melt the butter in a small saucepan. Skim off the foam, then slowly pour the clear melted butter into a large nonstick skillet set over moderately high heat, stopping when you reach the milky residue. Add the mustard seeds and cook until they begin to pop, about 30 seconds. Add the onion, ginger, garlic and coriander and cook over moderate heat until the onion is softened, about 5 minutes. Add the chile, ground dried mango, garam masala and potatoes and cook for 5 minutes, stirring and mashing gently. Remove the skillet from the heat, stir in the cilantro and season with salt and pepper.
Line a baking sheet with wax paper and dust it lightly with cornstarch. Working with 3 gyoza wrappers at a time and keeping the rest covered with plastic wrap, brush the edges with water. Spoon 2 teaspoons of the potato filling into the center of each wrapper. Fold the wrappers in half over the filling, pressing out any air bubbles and sealing the edges. Gently press the samosas so they stand upright with the seam on top, then transfer them to the prepared baking sheet. Repeat with the remaining wrappers and filling.
Heat 1 tablespoon of the oil in a large nonstick skillet. Add half of the samosas and cook over moderately high heat until sizzling and slightly browned. Add 1/4 cup of water, cover and steam until the wrappers are tender, about 2 minutes. Uncover and cook until the water is evaporated and the samosas are golden on the bottom, about 1 1/2 minutes longer. Transfer to a plate and repeat with the remaining oil, samosas and water. Serve with the tamarind sauce.